A data file usually gets damaged due to crashing of Outlook or exceeding the logical size limit of the file format. PST files that were created on Outlook versions 2000-2002 are limited to 2GB of size. Still, it usually gets crappy after 1.5GB. Later versions of Outlook can created upto 20GB of PST without (theoretically...) get stuck.

The Symptoms/Error Messages:
When the file gets damaged, you get several error while trying to run Outlook:

The Tools
We use two tools provided by Microsoft to deal with PST/OST problems:
SCANPST: used for the error scanning and fixing of PST and OST on some cases.
SCANOST: used for scanning and fixing of OST while connecting to the Exchange server.
the tools are located in the following path, depanding on your Office version: Office 2002-2003

C:\Program Files\Common Files\System\MSMAPI\1033* the number in the end can vary according to your Office setup localization.

Office 2007C:\Program Files\Microsoft Office\Office12

After running the tool, you need to select the data file for scanning. You can use the Browse button for easy navigation. the first scan might take some time to complete. the tool might seem as "not responding". I suggest you leave it running until it finishes without interapting (it's not the most stable tool I've used). after the scan finishes it suggest you to backup the file and then fix the problem. you can skip the backup if you wish, it usually don't make any additional problems.

When the scan finishes you can try running Outlook and see if it opens normally.
On some cases I needed to re-run the scan&fix process a couple of times and every time it found some more problems.